Japanese Patent Application Publication No. 2001-89222 discloses a technique for quickly warming a catalyst by performing fuel injection of a direct-injection injector once during a period from an intake stroke to a compression stroke and once during a period from an expansion (combustion) stroke to an exhaust stroke to make an air fuel ratio of the total amount of the fuel injection amounts in the two fuel injections leaner than the stoichiometric air-fuel ratio.
Japanese Patent Application Publication No. 2001-73913 discloses a technique of dividing a fuel injection amount for a direct-injection injector into two portions in order to increase exhaust gas temperature when the temperature is low requiring a catalyst to be quickly warmed. The fuel dividing ratio is set so that the exhaust gas temperature increases when the temperature of the catalyst is lower than a predetermined temperature, and so that CO and O2 concentration increases to lower the catalyst activation temperature when the temperature of the catalyst becomes higher than the predetermined temperature.
Japanese Patent Application Publication No. 2001-323834 discloses a technique of dividing a fuel injection amount of a direct-injection injector into two portions in order to increase exhaust gas temperature under the condition of a predetermined temperature or less at which a catalyst has to be quickly warmed. The intake air volume of an engine is increased so that the divided fuel injection amount does not become smaller than the minimum fuel injection amount of the injector.
Hitherto, as a technique of reducing emission immediately after start of the engine, there is a widely used technique of in-taking larger volume of air during fast idling than that in a normal idle state and, by retarding an ignition timing, thermal energy (not only temperature but also mass) of exhaust gas is increased to shorten activation time of the catalyst.
In an exhaust system for a high-performance engine (such as 4-2-1 exhaust system) producing high output and high torque, as its radiating area is large and heat capacity is large, the thermal energy of the exhaust gas is absorbed by the exhaust system, lowering the rate of heating the catalyst. Due to this, to shorten catalyst activation time, some vehicles employing the exhaust system for a high-performance engine use a scheme of increasing the exhaust gas temperature by providing a direct-injection injector to the engine and performing fuel injection in expansion and/or exhaust strokes (called expansion/exhaust stroke injection) in addition to a fuel injection in intake/compression strokes of a combustion cycle.
However, in the expansion/exhaust stroke injection, when cylinder gas temperature in the expansion/exhaust strokes is not high enough, sufficient re-burning cannot take place. As such, unburned HC (called slip HC) increases. In the worst case, the exhaust gas temperature goes down by latent heat of vaporization of the fuel in the expansion/exhaust stroke injection, and the catalyst activation takes longer time.
Consequently, as in the JPAP No. 2001-89222, the fuel injection in the expansion/exhaust strokes may be reduced so that the air-fuel ratio of the total amount of fuel becomes leaner. In practice, however, as the intake air volume during fast idling is not so large, even when a minimum amount of fuel for the direct-injection injector is injected in the exhaust/expansion strokes, the air fuel ratio of the total fuel hardly becomes leaner, and the slip HC cannot be reduced.
In the technique of the JPAP 2001-73913, the fuel injection during the period of the intake and compression strokes is divided into two, and the fuel dividing ratio is set on the basis of the catalyst temperature. However, the divided fuel injection in the intake and compression strokes cannot sufficiently shorten the catalyst activation time in the exhaust system for high-performance engine. Since a temperature sensor attached to the exhaust system is subjected to high temperature and water vapor. To make the sensor tolerable to heat and corrosion, the heat capacity of the sensor becomes large, causing a large delay in response. Consequently, when the control of the partial fuel injection is performed using a temperature sensor, a delay occurs in the timing of changing the dividing ratio of injection, making it hard to obtain proper catalyst temperature rising property.
JPAP No. 2001-323834 proposes a method for reducing slip HC during the catalyst warm-up control. In this method, the intake air volume is increased in accordance with the minimum fuel injection amount, but the fuel injection amount itself is not decreased. That is, since the HC amount injected is not reduced, the slip HC cannot be reduced.
As described above, there is a need for a scheme capable of shortening catalyst activation time while minimizing the slip HC circumventing the constraints of the minimum fuel injection amount of the direct-injection injector in a vehicle employing the exhaust system for high-performance engine.